A wireless local area network (WLAN) system works in an unlicensed spectrum, and users may share the spectrum on a channel. If multiple users send data at the same time may cause a collision that may interfere with each other. Therefore, in the WLAN system, a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism is used to avoid a collision. In an existing WLAN system, data packets contend, in a CSMA/CA mechanism, for sending. Carrier sense (CS) indicates that before sending a frame, any device connected to a medium needs to sense the medium, and only when it is determined that the medium is idle, the device can send the frame. Multiple Access (MA) indicates that multiple devices may access a medium at the same time, and a frame sent by one device may also be received by multiple devices. According to the CSMA/CA mechanism, an active collision avoiding manner rather than a passive sensing manner is used to resolve a collision problem, which can meet a requirement imposed when it is difficult to accurately sense whether a collision occurs. A working manner of the CSMA/CA mechanism is that when a device intends to send a frame and obtains by means of sensing that a channel is idle, after the channel remains idle for a time period, if the channel is still idle after the device waits for another random time period, the device submits data. Because a waiting time of each device is generated randomly, it is likely that there is a difference, so that a possibility of collision can be reduced. A probability of occurrence of a collision is related to how busy a system is. In the WLAN system, there is a hidden node problem, that is, when an access point (AP) such as AP1 sends data to a station (STA), and that station fails to sense the AP1. In this example another AP, AP2, may consider that a channel is idle and start to send data, thereby interfering with a receiving device STA. This problem cannot be resolved by using the CSMA/CA protocol.
Because of the hidden node problem, in the WLAN system, the request to send/clear to send (RTS/CTS) protocol is generally used to perform transmission protection. This protocol actually is that: a channel is reserved first before data is sent. When the AP1 sends data to the STA, the AP2 may also perform sending, thereby interfering with receiving of the STA. The RTS/CTS protocol defines that, before sending data, the AP1 first sends an RTS frame, and the STA returns a CTS frame after receiving the RTS frame. All APs or STAs receiving the RTS or CTS frame set a network allocation vector (NAV) according to an indication of the received RTS or CTS frame, where the NAV is a time corresponding to a sending time required by the AP1. These APs or STAs cannot send data within the NAV time. After sending an RTS frame and receiving a response of the STA, the AP1 obtains a sending opportunity, and the AP1 sends data to the STA within this time period, which is not interfered with by an AP or STA nearby.
In some cases, an AP may need to acquire a transmission opportunity of a time period, so as to exchange data with multiple STAs. In this time period, the AP may need to send data to the multiple STAs, and may also receive data from the multiple STAs.
The existing RTS/CTS protocol is applicable to only protection of data transmission between an AP and one STA. To protect transmission between an AP and multiple STAs, the AP may send an RTS frame to each of the multiple STAs sequentially, and the STAs sequentially return a CTS frame. However, this requires multiple groups of RTS/CTS interactions, which greatly increases an extra overhead of a system and reduces system efficiency.